Sensor networks have many applications, for example in security systems, environmental and industrial monitoring, military and biomedical applications. In many of these applications it is not possible to connect sensors using wires, for which reason the use of wireless sensor networks may be used.
The monitoring and detection of vehicles, humans and other similar objects is a complicated task since reliable identification and monitoring requires a high number of sensors placed in each other's proximity. For this reason arises a demand for a wireless sensor network, which consists of cheap sensors with low energy requirements.
In U.S. Pat. No. 6,208,247 compact wireless sensor stations are described, which can be connected to a network with other similar sensor stations and enable the measurement and analysis of vibration, infrared emission, sound or other signals specific to an intruder or a specific condition of the environment. Such sensor stations are embedded computers with low energy requirements and are equipped with at least one sensor, a microcontroller, wireless communication interface and an appropriate energy source. The network is formed using multi-hop ad-hoc networking principles, where the sensor stations also operate as routers and sensor stations can join and leave the network at runtime. Such sensor stations can be placed in selected locations manually or from vehicles, including deployment from aerial and water vehicles. The sensor stations are programmed to organize into a network that employs decentralized control, meaning that there is no requirement for a central control centre.
In U.S. Pat. No. 6,208,247 the sensor stations contain at least one appropriate sensor, by means of which analog signals are converted to digital signals and which selected power spectrum is compared to a sample spectrum stored in the sensor station (which may mean the existence or not existence of an alarm). As a result of the comparison the sensor station makes an appropriate decision—takes no action, wakes up the processor from the energy saving mode and executes an additional signal processing step; starts the communication module and forwards the spectrum, forwarding the unprocessed signal. The sensor stations can be reprogrammed if required, for example new sample spectrums could be loaded to the sensor station or the sensor stations could be programmed for a longer period of activity. For communication with other sensor stations a short range multihop TDMA communication scheme is used which allows reducing energy consumption and using a small number of channels for communication. The addition and removal of sensor stations is performed easily and automatically.
The described prior art solution has a range of shortcomings. One of the most critical aspects of wireless sensor networks is the energy consumption, most of the energy being consumed by the communication interface and the microcontroller. The prior art solution assumes that the sensor stations are constantly able to receive messages from nearby sensor stations which means that the sensor stations are constantly in a mode where they consume a substantial amount of power. The decisions are made in each sensor station based on the sensor readings of the sensor station, which means that in case of each alarm the data is forwarded, not taking into consideration the sensor readings acquired by other similar or dissimilar sensor stations, which potentially may increase the number of false alarms and also the amount of data sent out by the sensor stations and the amount of energy consumed by the network. Similarly the solution is designed to communicate the information to a central location, regardless if it was a false alarm or not. If the alarm was detected by several sensor stations the information is sent out to a central location autonomously by all such sensor stations.